Another possible answer is that the גמרא in חלק tells us that the עורב was one of the three who were משמש in the תיבה.
Perhaps the עורב knew this, and therefore he knew that the קיום המין was not dependent on his return. But either he knew that Noach wasn’t aware of this, (or he wasn’t aware of the ברית of קיום המין) so he told him that it would put the קיום המין in jeopardy if he sent him out of the תיבה.
But כלפי שמיא it was a תשובה נצחת because in reality the עורב already had perpetuated קיום המין in the תיבה!
UPDATE:
שוב ראיתי this from the חתם סופר: